Spin coating is a procedure that utilizes centrifugal forces created by a spinning substrate to uniformly spread a coating solution over a surface. This coating technique is relatively fast and efficient and is used in semiconductor wafer production.
In general, a spin mechanism is the heart of the spin coating process. The spin mechanism requires a holder for the wafer, generally, a vacuum chuck, a motor and a controller for spinning at a predetermined spin rate and time and a spin cup to catch excess fluids. A typical spin process consists of dispensing a coating solution onto a substrate mounted on the wafer chuck. The spin speed of the substrate is then increased to the predetermined rate for a predetermined period of time. During the spin time, the coating solution spreads across the substrate and most of the excess solution is forced off the edge. The wafer is then slowed at a given rate until it becomes stationary and can be removed from the chuck.
Generally, coating solutions with higher viscosity require a higher spin rate and spin coating solutions with less viscosity can be spun at a slower rate.
In general, the material to be made into the coating is dissolved or dispersed in a solvent and the coating solution thus comprised is spun off leaving a uniform layer for subsequent processing stages. Spin coating is known in the art for photo-resist for defining in patterns in microcircuit fabrication, magnetic disk coatings, compact discs, and for applying various anti-reflection and conductive coatings.
One area of particular concern with spin coating is edge effects. The edge of the wafer can be a source of difficulty in spin coating for several reasons. Surface tension effects may make it difficult for coating solution flowing radially outward on the wafer to detach from the wafer edge, thus, a small ridge of liquid can remain attached around the entire perimeter resulting in thicker coatings in this edge zone. In addition, if the wafer is not precisely round, the airflow over the protruding parts may result in non-uniformity in coating thickness. Increasing the coating solution viscosity can help to reduce edge effects, as can slowing the spin rate. These solutions to the edge effect problem are not entirely satisfactory, however. It is not always practical to thin the coating solution, and slower spin rates may result in longer processing times with a corresponding increase in expense.
Methods and apparatus providing the capability of performing backside rinsing without reintroducing contamination from the coating material would be useful and desirable in the art. Methods and apparatus capable of speeding the drying of spin coated semiconductor wafers would also be desirable in terms of savings in time and costs.